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Charlie Wood, "How Ancient Light Reveals the Universe's Contents", Abstractions Blog, Quanta Magazine, January 28, 2020, https://www.quantamagazine.org/how-the-cosmic-microwave-background-reveals-the-universes-contents-20200128/

A photograph of the infant cosmos reveals the precise amounts of dark matter and dark energy in the universe, leaving precious little room for argument.

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Smaller blobs reveal the cosmos’s composition. Their sizes, and how thick or thin they became, depended on the ingredients of the liquid, much as clam chowder splashes differently than chicken soup. In the primordial fluid, dark matter felt the pull of gravity, but not the electromagnetic push from light rays. Normal matter responded to both. Tesla Jeltema, a cosmologist at the University of California, Santa Cruz, explained that researchers can distinguish these two fluid components by comparing the intensity with which blobs of different sizes sloshed inward and outward — the relative heights of the peaks in the power spectrum.

But wouldn't radiation pressure felt by 'normal' matter slow down its contraction, but then speed up its expansion, relative to dark matter clumping and expansion? So that the differences 'cancelled out', making it impossible to distinguish between dark-matter-created 'blobs' in the CMB and baryonic-matter-created ones?

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"But wouldn't radiation pressure felt by 'normal' matter slow down its contraction, but then speed up its expansion, relative to dark matter clumping and expansion?"

This question seems to ignore the following quote: "Tesla Jeltema, a cosmologist at the University of California, Santa Cruz, explained that researchers can distinguish these two fluid components by comparing the intensity with which blobs of different sizes sloshed inward and outward — the relative heights of the peaks in the power spectrum." The key phrase is: "blobs of different sizes sloshed inward and outward".

The question notes that there are differences in behavior, but ignored the pattern of back-and-forth repetition.

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